1. Field of the Invention
The present invention relates to a fuel cell system for generating electrical energy by an electrochemical reaction between a fuel and an oxidizing agent. More particularly, the present invention relates to a fuel tank and a fuel cell system including the fuel tank.
2. Description of the Related Art
Fuel cells form an electricity generating system for generating electrical energy using an oxidation reaction of a fuel and a reduction reaction of an oxidizing agent gas. Fuel cells can be categorized as polymer electrolyte membrane fuel cells or direct oxidation membrane fuel cells according to the type of the fuel they use. In a polymer electrolyte membrane fuel cell, a fuel processing apparatus, such as a reformer, reforms a fuel into hydrogen gas, and the hydrogen gas is supplied to a stack. In a direct oxidation fuel cell, unlike in the polymer electrolyte membrane fuel cell, a fuel such as methanol (rather than hydrogen gas) is directly supplied to a stack.
In fuel cells, the hydrogen gas may be generated by a fuel reforming reaction. Also, the hydrogen gas may be generated by hydrolyzing a metal hydride compound. A fuel cell using the metal hydride compound and according to a first conventional embodiment has a structure where catalysts are added to a fuel solution that is a mixture of a metal hydride compound and water. A fuel cell using the metal hydride compound and according to a second conventional embodiment has a structure where the metal hydride compound and the water are supplied into a reaction container in which catalysts are stored. A fuel cell using the metal hydride compound and according to a third conventional embodiment has a structure where a catalytic solution is supplied into a reaction container in which the metal hydride compound is stored.
A fuel including sodium borohydride (NaBH4) may be used for the fuel cell using the metal hydride compound. During a hydrolysis reaction of the sodium borohydride (NaBH4), an alkaline material such as sodium is generated, as well as the hydrogen gas. If the alkaline material such as sodium is provided to a fuel cell stack together with the hydrogen gas, the output voltage of the generated electrical energy is deteriorated as shown in FIG. 8. Referring to FIG. 8, the deterioration of the output voltage of the fuel cell increases as the amount of the alkaline material such as sodium increases. In addition, the deterioration becomes worse as time passes. This is because the alkaline material such as sodium is adhered to a polymer electrolyte membrane such as Nafion disposed in the stack, and prevents (or blocks) an electrochemical reaction between the hydrogen gas and oxidizing agent gas.
As such, in a conventional fuel cell using the metal hydride compound, research to eliminate the alkaline material such as sodium that is generated together with the hydrogen gas has been made.
In Japanese Patent Laid-Open Publication No. 05-137979, a hydrogen separator is disposed in order to eliminate the alkaline material such as sodium. The hydrogen separator is formed of a Pd sheet or a Pd alloy sheet. However, since Pd metal is expensive, it is difficult to commercialize the fuel cell using the Pd sheet or Pd alloy sheet.
In Japanese Patent Laid-Open Publication No. 2006-314944, a polyimide membrane as a hydrogen separator is disposed in order to eliminate the alkaline material such as sodium that is generated together with the hydrogen gas. However, the polyimide membrane is chemically and/or structurally weak with respect to a chemical reagent such as an alkaline solution, and hydrogen separating speed of the polyimide membrane is low.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.